mhi: core: Finish pending reg writes before entering suspend

It is possible that the write offload worker gets a chance to
run after PCIe link is handed off to DRV subsystem. DRV
subsystem can enter L1SS sleep while work handler is trying to
access MHI register. This results into NOC error. Fix this
issue by flushing work before handing off PCIe link to DRV
subsystem or link enters D3 cold.

Change-Id: I83e33b3753b62bf13a7f50af52c951b7a09a32e7
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>